The present invention relates to an image display device such as a liquid crystal display device, to an image capture device such as a flat-panel image sensor, and to an active matrix substrate used in the foregoing devices and a method of manufacturing such an active matrix substrate.
Making the most of their superior characteristics including having a large surface area, and being thin and light weight, active matrix substrates can be used not only in image display devices such as liquid crystal display devices, but also in image capture devices such as flat-panel X-ray image sensors.
The following will explain the structure of a conventional active matrix substrate with reference to FIGS. 6 through 8. FIG. 6 is a plan view of a conventional active matrix substrate, showing one of a plurality of pixels provided in the active matrix substrate. Such an active matrix substrate is used to improve aperture ratio in the liquid crystal display devices such as those disclosed in Japanese Unexamined Patent Publication No. 58-172685/1983 (Tokukaisho 58-172685, published on Oct. 11, 1983) and U.S. Pat. No. 5,953,084 (issued on Sep. 14, 1999), and is structured as follows.
A plurality of scanning lines 52 and signal lines 56 are arranged in a lattice form on a glass substrate 51 (see FIGS. 7 and 8), and a switching element and a pixel electrode 57 are provided for each area where a scanning line 52 and a signal line 56 cross. Each of the switching elements is a thin-film transistor (hereinafter referred to as xe2x80x9cTFTxe2x80x9d) 60, in which a gate electrode 55 is connected to the signal line 52, a source electrode 61 is connected to the signal electrode 56, and a drain electrode 63 is connected to the pixel electrode 57 via a drain line 63a and a contact hole 57a. Further, below the contact hole 57a is provided an auxiliary capacitance line 53.
FIGS. 7 and 8 are cross-sectional views of the foregoing conventional active matrix substrate, taken at lines Dxe2x80x94D and Exe2x80x94E, respectively, of FIG. 6. On the insulative glass substrate 51 are provided the scanning line 52 (see FIG. 6), the gate electrode 55, which is a branch line of the scanning line 52, and the auxiliary capacitance line 53. On the surfaces of these elements oxidation films 52a and 53a (anodic oxidation (AO) films) are provided by anodic oxidation. Then a gate insulating film 54 is provided on the oxidation films 52a and 53a on the scanning line 52 and the auxiliary capacitance line 53, and on areas of the glass substrate 51 where the scanning line 52 and the auxiliary capacitance line 53 are not provided.
Then, on areas of the gate insulating film 54 lying above the gate electrode 55, a TFT 60, which is a switching element made up of a semiconductor domain 64 and a contact layer 65, is provided for each pixel. The source electrode 61 of the TFT 60 is connected to the signal line 56, which is provided on the gate insulating film 54 extending in a direction perpendicular to a direction in which the scanning line 52 extends. Further, the drain electrode 63 of the TFT 60 is connected to the drain line 63a provided on the gate insulating film 54.
Over the foregoing elements is provided a protective film 58 which covers the TFT 60 (including the source and drain electrodes 61 and 63), the signal line 56, the drain line 63a, and the gate insulating film 54. Then an inter-layer insulating film 59 and the pixel electrode 57 are provided over the foregoing elements. The pixel electrode 57 is electrically connected to the drain line 63a via a contact hole 57a which penetrates the inter-layer insulating film 59 and the protective layer 58. Further, the drain line 63a lies opposite the auxiliary capacitance line 53, but separated therefrom by the gate insulating film 54 and the oxidation film 53a, thus forming an auxiliary capacitance 62.
The following will explain a flat-panel X-ray image sensor and a liquid crystal display device which use the foregoing active matrix substrate.
In a flat-panel X-ray image sensor, a device intended to replace X-ray photograph devices which use conventional photosensitive photographic film, an image is formed based on a two-dimensional distribution of X-ray quantities incident on a flat panel of the image sensor. When using such a device, an X-ray source is provided separately, and an object to be photographed is placed between the X-ray source and the image sensor.
When an active matrix substrate is used in such a flat-panel X-ray image sensor, as disclosed in Japanese Unexamined Patent Publication No. 4-212458/1992 (Tokukaihei 4-212458, published on Aug. 4, 1992), a photoelectric conversion layer, for converting X-rays to electrical charges, is provided on the pixel electrodes 57, and the pixel electrodes 57 are used as charge-collecting electrodes. The photoelectric conversion layer is made of a semiconductor element, and this semiconductor element is provided by film formation directly on the pixel electrode 57, or by laminating thereon a semiconductor element formed separately.
The foregoing flat-panel X-ray image sensor provided with an active matrix substrate operates as follows. A DC current is applied between the pixel electrode 57 and a counter electrode provided above the photoelectric conversion layer. The TFT 60 is OFF except when reading an image, and a charge produced in the photoelectric conversion layer by X-rays incident thereon is collected in the auxiliary capacitance 62 via the pixel electrode 57. Reading of this charge is performed by selecting the corresponding pixel using the scanning line 52, and allowing the charge accumulated in the auxiliary capacitance 62 to flow to the signal line 56 via the TFT 60. Charges read out are amplified by a circuit, such as an operational amplifier, connected to the end of the signal line 56. Then an image is formed based on the distribution of charge quantities read out from all of the pixels.
When, on the other hand, the foregoing active matrix substrate is used in a liquid crystal display device, a counter electrode is provided opposite the pixel electrodes 57, with a liquid crystal layer therebetween. Then, by applying a potential difference between a pixel electrode 57 and the counter electrode, light passing through the liquid crystal layer is subject to a rotation of its plane of polarization corresponding to the potential difference. The direction of the plane of polarization of the light determines a quantity of light passing through a polarizing plate provided externally, thus forming an image by the intensity of light in each pixel.
In the active matrix substrate in this case, in a pixel selected by the scanning line 56, a potential is written to the pixel electrode 57 from the signal line 56 via the TFT 60. This produces the foregoing voltage between the pixel electrode 57 and the counter electrode.
Electrostatic capacitance parasitic in the lines provided in the active matrix substrate greatly influences the performance of the active matrix substrate. This electrostatic capacitance not only causes delay in transmission of signals inputted to the ends of these lines and of data from the pixels, but also causes the potential of non-target pixels and lines to fluctuate, and makes the potential of target lines susceptible to external influence. A further problem with this electrostatic capacitance is that it impairs the quality of images captured or displayed by the device incorporating the active matrix substrate.
In a flat-panel X-ray image sensor, image data is formed on the basis of charges accumulated in the pixel electrodes 57, read out through the signal lines 56. For this reason, electrostatic capacitance (signal line capacitance) between the signal lines 56 and the scanning lines 56 and auxiliary capacitance lines 53 increases the time necessary to read out the charges, and increases the noise component of the charges read out. As a result, quality of the captured image is impaired.
Particularly in a flat-panel X-ray image sensor, which forms an image from weak X-rays, it is necessary to read charges of small quantity accumulated in the pixel electrodes 57, and the influence of the foregoing electrostatic capacitance is especially serious.
However, in the conventional art, in which the signal lines 56 are provided on the gate insulating film 54, structurally, the interval between the signal lines 56 and the scanning lines 52 and that between the signal lines 56 and the auxiliary capacitance lines 53 tend to become too small. This tends to increase the signal line capacitance value, which is likely to impair the quality of the captured image.
Further, in a flat-panel X-ray image sensor, since the pixel electrode 57 is used for charge accumulation, the potential thereof tends to fluctuate more than in a liquid crystal or other display device. In order to prevent fluctuation in potential of the pixel electrode 57 from causing malfunction of the TFT 60, it is necessary to suppress this fluctuation as much as possible. For this reason, the auxiliary capacitance 62 connected to the pixel electrode 57 must have a much greater capacitance value than in a liquid crystal display device. However, since there are limits to how much the auxiliary capacitance value can be increased by increasing the width or surface area of the auxiliary capacitance lines 53, decreasing the thickness of the gate insulating film 54 or the oxidation film 53a is considered an effective means of increasing the auxiliary capacitance value.
However, in the conventional structure, decreasing the thickness of the gate insulating film 54 or the oxidation film 53a has the disadvantage of greatly increasing the signal line capacitance value, and thus there are limits to how thin these members can be made. Thus, a drawback with the conventional structure was that is was difficult to increase the auxiliary capacitance value.
Liquid crystal display devices, like flat-panel X-ray image sensors, also have the problem that in the conventional structure the signal line capacitance value tends to become large. Liquid crystal display devices can be expected to become even more high-definition in the future, but increasing definition generally increases the capacitance value of each line, and display quality suffers. Thus, further increase in definition in the future calls for a new active matrix substrate structure capable of reducing signal line capacitance value, which greatly influences display quality.
Further, in order to obtain bright display in a liquid crystal display device, it is especially important to increase aperture ratio. However, in an active matrix substrate with the conventional structure, the auxiliary capacitance lines 53, which block light, occupy a high proportion of the total surface area of the pixel, and thus contribute to reduced aperture ratio when used in a transmittive liquid crystal display device. Further, it is possible to secure the necessary auxiliary capacitance value while improving aperture ratio by decreasing the width of the auxiliary capacitance lines 53 and decreasing the thickness of the gate insulating film 54 or the oxidation film 53a, but, just as in the foregoing flat-panel X-ray image sensor, this tends to increase the signal line capacitance value.
In response to this problem, the conventional art attempted to reduce the signal line capacitance value by, for example, forming a protective film covering the semiconductor domain 64 of the TFT 60 and the glass substrate 51, and then forming on this protective film the source electrode 61, the drain electrode 63, the signal line 56, the pixel electrode 57, etc., as disclosed in Unexamined Japanese Patent Publication No. 60-160173/1985 (Tokukaisho 60-160173, published on Aug. 21, 1985).
In such a structure, however, since contact between the source and drain electrodes 61 and 63 on the one hand and the semiconductor domain 64 on the other is likely to become unstable, the protective film cannot be made too thick, and thus provides insufficient reduction of the signal line capacitance value. Further, the foregoing structure gives no thought to an auxiliary capacitance line 62, and if an auxiliary capacitance line 62 is to be provided, the foregoing structure and the process for manufacturing it become complicated.
As discussed above, in order to reduce noise in a captured image and increase reliability in a flat-panel X-ray image sensor, and to secure display quality and aperture ratio in a high-definition liquid crystal display device, a structure for an active matrix substrate is called for which, first, enables reduction of the signal line capacitance value, and, second, prevents increase of the signal line capacitance value when the thickness of the gate insulating film 54 or the oxidation films 52a and 53a is decreased (when the auxiliary capacitance value is increased).
It is an object of the present invention to provide an active matrix substrate having a small signal line capacitance value, and to provide an active matrix substrate in which a signal line capacitance value is only slightly increased by increasing an auxiliary capacitance value, in order to realize an image capture device or image display device capable of obtaining a higher quality captured or displayed image.
In order to attain the foregoing object, an active matrix substrate according to the present invention is made up of switching elements, each switching between a source electrode and a drain electrode based on a signal supplied to a gate electrode; scanning lines connected to the gate electrodes; signal lines connected to the source electrodes; and pixel electrodes connected to the drain electrodes; in which a substrate is provided with a layer which forms the scanning lines; a layer, provided above the layer forming the scanning lines, which forms the source electrodes; a layer, provided above the layer forming the source electrodes, which forms the signal lines; and an insulating layer, provided between the layer forming the source electrodes and the layer forming the signal lines; and the scanning lines and the signal lines are provided on opposite sides of the insulating layer from each other.
In the foregoing structure, the substrate is provided with a layer which forms the scanning lines; a layer, provided above the layer forming the scanning lines, which forms the source electrodes; and a layer, provided above the layer forming the source electrodes, which forms the signal lines. Further, an insulating layer is provided between the layer forming the source electrodes and the layer forming the signal lines. In addition, the scanning lines and the signal lines are provided on opposite sides of the insulating layer from each other, extending, for example, in intersecting directions.
The conventional active matrix substrate was structured such that the signal lines and scanning lines were provided on opposite sides of a gate insulating film (which was provided on the scanning lines and the gate electrode). In this structure, the thickness of the gate insulating film was determined in accordance with the specifications of the switching element, and since the electrostatic capacitance value of the gate insulating film is based on the thickness of this film, it was difficult to set a smaller value. For this reason, in areas where a signal line and a scanning line are opposite one another on opposite sides of the gate insulating film, the capacitance value of a signal line capacitance (parasitic capacitance) arising between the signal line and the scanning line increases, thus giving rise to the problem discussed above.
In the structure of the present invention outlined above, however, the signal lines and scanning lines are separated by the insulating layer, which is provided between the layer forming the source electrodes and the layer forming the signal lines. Consequently, the interval between the signal lines and scanning lines separated by the insulating layer can be set to a greater value than the thickness of the gate insulating film in the conventional structure. Thus the capacitance value of the signal line capacitance arising between the signal lines and the scanning lines can be reduced in comparison with the foregoing conventional structure.
Since the foregoing insulating layer is provided between the layer forming the source electrodes and the layer forming the signal lines, it is independent of the specifications of the switching elements, unlike a gate insulating film. Accordingly, the insulating layer can be formed in such a way that the signal line capacitance value is sufficiently reduced.
Specifically, the signal line capacitance value can be sufficiently reduced by forming the insulating layer with a sufficient thickness, and using a material having a small relative dielectric constant.
Further, some conventional active matrix substrates were structured so as to provide an insulator such as a protective film between the semiconductor domain of the switching element and the source electrode. In this case, since, as discussed above, contact between the source electrode and the semiconductor domain tends to become unstable, it is difficult to make the insulator sufficiently thick, and thus difficult to reduce the signal line capacitance value.
In the structure of the present invention outlined above, however, since the layer forming the source electrodes and the layer forming the signal lines are provided separately, it becomes possible to form the source electrodes in close contact with the main body of each switching element (e.g., the semiconductor domain). Accordingly, the thickness of the insulating layer can be increased sufficiently without adversely affecting the functioning of the switching element.
In addition, the source electrode and the signal line can be connected by, for example, a contact hole formed in the insulating layer. Further, since a domain large enough to allow formation of this connection area can be secured away from the switching element, connection will not become unstable.
Accordingly, the foregoing structure makes it possible to provide an active matrix substrate with good switching element functioning, and in which a signal line capacitance arising between the signal lines and the scanning lines has a small capacitance value.
Further, the present invention discloses a method of manufacturing an active matrix substrate in which pixel electrodes are made from the same layer as that of signal lines, and the method includes the step of forming the pixel electrode and the signal line by patterning of a single layer.
In the foregoing method, the signal lines and pixel electrodes are formed by patterning of the same layer (film). In other words, by patterning of a single conductive film made of a single material, the signal lines and the pixel electrodes can be formed in the same process.
Accordingly, the active matrix substrate according to the present invention can be manufactured by modifying part of the pattern of a pattern mask used in forming the pixel electrodes in manufacturing a conventional active matrix substrate. Consequently, in manufacturing the active matrix substrate according to the present invention, it is possible to avoid complication of the manufacturing process, increase of the number of manufacturing steps, etc.
Further, by forming the source electrodes and drain electrodes by patterning of the same layer (film), it is possible to further simplify the manufacturing process.
In order to attain the foregoing object, an active matrix substrate according to the present invention includes signal lines connected to switching elements, scanning lines attached to the switching elements, and a resin material provided between the signal lines and the scanning lines.
In this structure, the signal lines and scanning lines are provided in the active matrix substrate on opposite sides of the resin material, extending, for example, in intersecting directions.
Resin materials generally have a lower relative dielectric constant than, for example, inorganic materials. Accordingly, by providing the signal lines and the scanning lines on opposite sides of a resin material, the capacitance value of an electrostatic capacitance arising between the signal lines and the scanning lines can be reduced. As a result, the signal line capacitance value can be reduced.
Incidentally, in an active matrix substrate provided with auxiliary capacitances, a resin material is preferably provided between the auxiliary capacitance lines forming the auxiliary capacitances and the signal lines. In this way, the signal line capacitance value can be further reduced.
In order to attain the foregoing object, another active matrix substrate according to the present invention is made up of switching elements, switching between a source electrode and a drain electrode based on a signal supplied to a gate electrode; scanning lines connected to the gate electrodes; signal lines connected to the source electrodes; and pixel electrodes connected to the drain electrodes; in which a substrate is provided with a layer which forms the signal lines; a layer, provided above the layer forming the signal lines, which forms the gate electrodes; a layer, provided above the layer forming the gate electrodes, which forms the scanning lines; and an insulating layer, provided between the layer forming the gate electrodes and the layer forming the scanning lines; and the scanning lines and the signal lines are provided on opposite sides of the insulating layer from each other.
In the foregoing structure, the substrate is provided with a layer which forms the signal lines; a layer, provided above the layer forming the signal lines, which forms the gate electrodes; and a layer, provided above the layer forming the gate electrodes, which forms the scanning lines. Further, an insulating layer is provided between the layer forming the gate electrodes and the layer forming the scanning lines. In addition, the scanning lines and the signal lines are provided on opposite sides of the insulating layer from each other, extending, for example, in intersecting directions.
In this structure, as in the structure described above (in which a layer forming the scanning lines, a layer forming the source electrodes, and a layer forming the signal lines are provided on the substrate in that order), the scanning lines and the signal lines are provided on opposite sides of the insulating layer (which is provided between the layer forming the gate electrodes and the layer forming the scanning lines). Consequently, the capacitance value of the signal line capacitance arising between the scanning lines and the signal lines can be reduced.
In addition, the gate electrode and the scanning line can be connected by, for example, a contact hole formed in the insulating layer. Further, since a domain sufficient to form this connection area can be secured away from the switching element, connection will not become unstable.
Accordingly, this structure makes it possible to provide an active matrix substrate with good switching element functioning, and in which a signal line capacitance arising between the signal lines and the scanning lines has a small capacitance value.
In order to attain the foregoing object, a flat-panel image sensor according to the present invention is made up of the foregoing active matrix substrate provided with pixel electrodes, and a photoelectric conversion layer electrically connected to the pixel electrodes of the active matrix substrate.
This structure provides a flat-panel image sensor in which pixel electrodes function as charge-collecting electrodes. Further, this structure provides a flat-panel image sensor which includes the foregoing active matrix substrate having a small signal line capacitance value. Consequently, it is possible to suppress increase of the time needed to read the charges due to a large signal line capacitance, and to suppress superimposition of noise on the signal of the signal line due to the influence of the scanning lines, auxiliary capacitance lines, pixel electrodes, etc.
Additional objects, features, and strengths of the present invention will be made clear by the description below. Further, the advantages of the present invention will be evident from the following explanation in reference to the drawings.